It's a 1985 model so It's unlikely to be at the fill pipe. I'll have to check at the sink this weekend. On a similar note I have three air vent holes on the hull. One for the holding tank, one for the fuel tank, both on the port side. But I have one more at the stern. Anybody have any idea what that's for? Thought it might have been for the water tank, except its a fair distance from the tank and seemed to be an unnecessary long run. The answers I've received would confirm that's it's not for the water tank. So what's it for?